The evolvement of communication technology, particularly IP-based communication technology and end user terminals, has enabled versatile communication possibilities and introduction of different services. More and more often services are implemented using primitives provided by SIP (Session Initiation Protocol) which is not vertically integrated into a communications system but a tool to build a multimedia architecture. More precisely, SIP is an IETF defined application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, multimedia conferences, and PoC (Push to talk over Cellular) sessions, for example. For messaging services, SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) using SIP and existing implementations of SIP to provide presence and instant messaging service is being defined in IETF. OMA (Open Mobile Alliance) also defines IM (Instant Messaging) enabler based on SIP/SIMPLE protocols. SIMPLE defines two modes of instant message exchange: page-mode and session-mode. The page-mode uses SIP MESSAGE method by which a page-mode instant message is sent and where, at a protocol level, a subsequent instant message is not related to the preceding one: each immediate message, even a reply to a previous message, is considered as an independent transaction. Thus, the SIP MESSAGE method resembles a conventional email or short message service. The session-mode uses SIP for signaling and session establishment and MSRP (Message Session Relay Protocol) for carrying a series of instant messages after a session has been established. Below, the combination is simply called an MSRP mechanism. In other words, an MSRP mechanism provides chat-type messaging, called session-mode messaging.
A problem arises when a user wants to send a large page-mode message. The SIP MESSAGE method may use either UDP or TCP transport. TCP provides a reliable transport method also for large messages, but TCP transport cannot always be guaranteed for the SIP MESSAGE method. If UDP is used for sending a large message, packets larger than UDP maximum size are fragmented and may not arrive in the right order at the recipient. In addition, even if TCP could be guaranteed, another problem relating to congestion control remains. Since the SIP MESSAGE method is part of SIP session control signaling, a message is sent and received using the very same resource as used by the SIP signaling. For a user terminal this means that the actual SIP signaling may be blocked for the time the large message is being sent or received in the user terminal. The above-mentioned resource for the SIP signaling may be a general-purpose PDP (Packet Data Protocol) context or a dedicated signaling PDP context in case of the GERAN (GSM/EDGE Radio Access Network) and/or UTRAN (UMTS Terrestrial Radio Access Network) systems, for example. In other systems, the resource may be a reserved and/or dedicated bandwidth for signaling purposes, for example. In addition to the SIP signaling being blocked, a further problem relating to loading of SIP proxies may arise. As a page-mode message conventionally uses the SIP MESSAGE method, all messages using the SIP MESSAGE method are transmitted through the SIP proxies. Thus, the large size page-mode messages transmitted through the SIP proxies may cause a severe decrease in the performance of the SIP proxies, resulting in both effectively blocking all SIP signaling and decreasing the overall performance of the SIP network. Therefore, in some cases, the SIP MESSAGE method is not feasible to be used for a large size message.
One solution is that when the message size exceeds a certain limit, instead of the SIP MESSAGE method the MSRP mechanism is used. However, the MSRP mechanism is for a session-mode messaging service, not for page-mode messaging. Additionally, received page-mode messages can be deferred and are stored into a messaging inbox, wherefrom the user can read them when it is convenient, but in the session-mode messaging a received message is opened by the user terminal and shown to the user to facilitate a dialog. Thus, from the receiver's point of view, no page-mode messages can be received when the MSRP mechanism is used.